(1) Field of the Invention
This invention relates to a power transmission apparatus which is utilized for example as a central differential in a four-wheel drive vehicle.
(2) Explanation of Prior Art
Differential devices which are capable of transmitting a differential rotation from an engine through a transmission to a front drive shaft and a rear drive shaft are well known. Such differential device allow a difference in rotation between the front wheels and the rear wheels at the time when the vehicle is negotiating a curve, thus enabling a smooth run.
However, with this known of differential device, when either one of the front or the rear wheels has bogged down in muddy ground, it begins to slip and to rotate a high speeds under substantially no load. Thus, the power of the engine is consumed by the high speed slip rotation of the bogged down wheel. For this reason, there is a problem in that difficulty occurs in getting the vehicle out of the muddy ground. In order to solve such a problem, in recent years, a differential device has been provided with means to limit differential. This means to limit differential, operates when the differential rotation exceeds a predetermined value, thus allowing a vehicle to get out of muddy ground by transmitting power to the wheels having firmer ground contact.
A known differential device having such means to limit differential is shown in FIG. 1. This device comprises a transmission shaft 1 which is connected to an output shaft of a transmission and which transmits engine power, and driven transmission shafts 3, 5 which are connected in a relative manner and which receive power transmission from the transmission shaft 1. A base plate 7 is fixed to the transmission shaft 1 and planetary gears 9 are provided freely rotatably on the base plate 7. A casing 11 is provided on the driven transmission shaft 3, and an internal gear 13 which meshes with the planetary gears 9 is provided circumferentially at the inside surface of the casing 11.
The driven transmission shaft 5 is hollow and fitted onto the transmission shaft 1, with a sun gear 15 which meshes with the planetary gears 9 provided at the outer periphery of the end portion thereof. The planetary gears 9, the internal gear 13 and the sun gear 15 form a differential device 17, and the rotation of the transmission shaft 1 is transmitted to the driven transmission shafts 3, 5, freely differentially. In this case, the rotation of the driven transmission shaft 3 is transmitted to a rear differential (not shown) and the rotation of the driven transmission shaft 5 is transmitted to a front differential (not shown) through a chain 16 and a drive shaft 18.
The means to limit differential which has been added to the differential device 17 comprises a plurality of flanged plates 19 projectingly provided on the driven transmission shaft 5, fin-like plates 21 fitted between the plates 19 with certain clearance therebetween, and silicone oil 23 which has been filled in a liquid-tight state inside the casing 11. Accordingly, for instance, in case there is a relative rotation between the driven transmission shafts 3, 5, namely, in case a differential rotation is caused in the planetary gear 9 due to either one of the front wheels being bogged down in muddy ground, the silicone oil between the flanged plates 19 and the fin-like plates 21 effects resistance due to its viscosity, and thus limits this differential rotation.
However, in this kind of prior art device, since the construction is such that the limitation of the differential is effected only by the shear action of the silicone oil, as shown in FIG. 3, the rising characteristic of the limitation torque caused by the shear resistance against differential speed of rotation is inferior, and also the value of the limiting torque produced is small. As a result, there was a problem that the limiting of the differnetial could not be effectively carried out. Of course, if the number of flanged plates 19 and fin-like plates 21 is increased, the limitation torque can more or less be increased. However, in such a case there are problems that the device will become bulky and the cost of the device will also become prohibitive. Further, there is a problem that, when the temperature of the viscous fluid rises by the differential rotation, the viscosity of the viscous fluid decreases, thereby adversely affecting the ability to obtain sufficient transmitted torque.